The research proposed here is aimed at discovering the genetic and ontogenetic causes of opiate addiction. The etiology of susceptibility to addiction, behavioral and physiological dependence on morphine will be measured behaviorally first, in commercial inbred strains of mice and then in stock strains hybridized from the commercial strains on the basis of their addictability. Concurrently, behavioral experiments will be done to refine the indices of addiction and addictability and morphological analyses of major organ systems will be conducted to insure against the production of severely aberrant strains. The program or research whose beginning is proposed here is one which will eventually lead to a strict genetic, environmental and developmental experiments directed toward discovering the contribution of genotype, environment, and genotype-environmental interactions to opiate addiction.